Integrated semiconductor circuits and structures, particularly systems or arrays having simple structures or devices each of which represents a binary digit of information, as in read only memories (ROM) or in programmable read only memories (PROM) have achieved high device or cell densities.
In known bipolar PROM cells, a so-called fusible diode method is used to program the memory wherein an emitter-base junction of a device is reverse biased at a high current inducing metal to become locally heated, producing a short to the P type conductivity base region of the device. Some inherent disadvantages of this method include the need for high current to program the memory and for isolation around each cell. Furthermore, the junction between the base region and the epitaxial layer produces a relatively high capacitance which determines the delay of the memory cell. Other types of programmable structures and PROM cells are also well known.
In, e.g., U.S. Pat. No. 4,152,627, filed June 10, 1977, there is disclosed a PROM having cells which include a diode connected to a fusible link.
U.S. Pat. No. 3,525,020, filed May 15, 1967, illustrates a semiconductor matrix having NPN transistors connected to selected vertical and horizontal lines.
U.S. Pat. No. 4,070,654, filed Sept. 13, 1976, describes a ROM having an array of Schottky barrier diodes programmed by contact openings formed at predetermined locations in an insulating film with an aluminum strip extending over the openings.
In U.S. Pat. No. 3,555,365, filed May 29, 1968, there is disclosed a memory having an array of Schottky diodes wherein selected diodes are isolated by melting bridges with a current of substantial magnitude.
Commonly assigned U.S. Pat. No. 3,641,516, filed by P. P. Castrucci et al. on Sept. 15, 1969, and also U.S. Pat. No. 3,781,825, filed May 10, 1971, describe memories including programmable elements having two series-connected and opposed diodes. To effect programming, selected diodes operated at high reverse bias voltages are short circuited.